This invention relates to an optical transmission system and, more particularly, to an optical transmission system in which one or more branch stations are connected to a main optical transmission line.
There are two different optical submarine cable systems for connecting branch stations to trunk lines, namely, the simple double landing system serially connecting main stations, and branch stations and the simple underwater branching system providing one branching apparatus for each branch station in a trunk line.
FIG. 33 is a block diagram roughly sketching the general construction of the simple double landing system. Two trunk stations 10a, 10b and two branch stations 10c, 10d are provided. The trunk station 10a and the branch station 10c are connected by an optical fiber cable 12 having a pair of optical fiber lines (hereinafter called optical fibers), the branch station 10c and the branch station 10d are connected by an optical fiber cable 14 having a pair of optical fibers, and the branch station 10d and the trunk station 10b are connected by an optical fiber cable 16 having a pair of optical fibers. One of optical fibers in the optical fiber 12 is the upward line, and the other is the downward line.
FIG. 34 is a block diagram roughly sketching the general construction of the simple underwater branching system. In this system, a trunk cable 22 having two pairs of optical fibers connects two trunk stations 20a, 20b. Two branch stations 20c, 20d are connected to one of two pairs of optical fibers of the trunk cable 22 by branching units 28, 30 via branching cables 24, 26 each having two pairs of optical fibers. The other of two pairs of optical fibers of the trunk cable 22 not connected to the branching units 28, 30 is used for communication between the trunk stations 20a, 20b whereas the pair of optical fibers connected to the branching units 28, 30 (and hence to the branching cables 24, 26) is used for transmission to and from the branch stations 20c, 20d. 
In the conventional simple double landing system, a trouble in any of the optical fiber cables 12, 14, 16 connecting stations, such as breakage, malfunction of an optical repeater, or the like, thoroughly disables communication between stations on both sides of the location of trouble. Moreover, in the simple double landing system, since optical signals used for communication between the trunk stations 10a, 10b travel through the branch stations 10c, 10d, a delay of communication between trunk stations is inevitable.
In the simple underwater branching system, since the trunk stations 20a, 20b are connected directly, transmission delay is small, and there is no particular reason of delay in communication between trunk stations. A problem lies in that any trouble in the branching cable 24 or 26 disables transmission to and from a corresponding branch station 20c or 20d. There is no obstacle for transmission between the trunk stations 20a, 20b unless any trouble occurs in the trunk cable 22.
If the amount and importance of communication to the branch stations 20c, 20d are less than those of communication between the trunk stations 20a, 20b, the construction of the system shown in FIG. 34 would be acceptable. However, the importance of the branch stations 20c, 20d increases, the conventional construction is insufficient.
That is, it has become more and more important to guarantee communication lines to branch stations as well, and a demand for techniques dealing with troubles in optical communication lines and supply lines.
It is therefore an object of the invention to provide an optical transmission system satisfying such requirements and having signal transmission lines to branch stations which are durable against troubles.
According to the invention, there is provided an optical transmission system having a plurality of branching units on a main transmission line, and connecting individual branching units to a branch station by individual branching cables and to each other by an inter-unit connecting cable. Each branching unit has means for transferring a signal from another station to the branch station via the branching cable or to the adjacent branching unit via the inter-unit connecting cable, and means for transferring signal from the branch station to another station or to the adjacent branching unit via the inter-unit connecting cable.
Thus, by using double communication lines to each branch station, the communication path with the branch station is made more reliable.
A power supply line may be provided for each branching cable to prevent generation and entry of noise light.
Each branching unit may be made by using optical elements having no movable portion, such as demultiplexing means, multiplexing means, coupler, optical circulator, and so forth, to construct a highly reliable branching unit with less troubles and to realize a highly reliable optical transmission system.
On the other hand, the transmission loss can be reduced by using a selective switch.
When each optical transmission line of each inter-unit connecting cable is used in a unidirectional mode, branching units can be disposed more distant, and the probability of simultaneous damages to a plurality of branching cables can be decreased.
Light detector/switch controller may be provided in each branching unit to control the switching action of an optical switch inside. Thus, the flow of optical signals in each branching unit is switched autonomously without the aid from a branch station or a trunk station.